In ships having the usual shape with a single main propeller, the flow to the main propeller is often somewhat uneven. This uneven inflow can produce vibrations in the propeller and drive system. It is also possible to generate vibrations in the hull, commencing with the plating of such hull located above the main propeller, namely in the area of the counter. Further, the individual blades of the propeller can generate vibrations. The unevenness of the propeller inflow also acts to diminish the propeller efficiency.
It is known to oppose these actions by a special shape of the ship's hull, through low friction flow surfaces or through additional propellers in the flow stream.
It is also known to arrange a control propeller behind the main propeller pivotable with the rudder. This, however, increases the unevenness of the flow stream and serves solely to improve control.
It is also known to arrange a supplemental propeller in relation to the direction of travel closely in front of the upper portion of the main propeller. In such an arrangement, however, the low pressure zone of the main propeller and the high pressure zone of the supplemental propeller adversely influence one another and this results in a considerable loss of efficiency. Also it is not possible to use this additional propeller as a control assist inasmuch as in the known arrangements, the direction of rotation of the two propellers is opposite.
Finally it is known to arrange a supplemental propeller pivotally about a vertical axis and in the direction of travel behind the main propeller and behind the rudder. The rudder has a suitable recess in its upper edge. Since this supplemental propeller lies in the flow stream of the main propeller, it must have a very high rotational speed to produce any additional propulsion when the main propeller operates, and the propeller blades must have a high pitch corresponding to the high inflow speed. This pivotable supplemental propeller can be used to maneuver, but for this purpose a small rotational speed and a low pitch for the propeller blades is needed. Therefore, it is possible to design this supplemental propeller either for a high service speed (flow stream of the main propeller) or for a smaller inflow speed during maneuver. For this reason, the known supplemental propeller has been arranged retractably and extendably, in order not to hinder the other propeller during a travelling condition. It would also be possible to equip the supplemental propeller with adjustable propeller blades. However, both measures, the extendability and the blade adjustability, are very expensive and especially so if the supplemental propeller is pivotable about a vertical axis for maneuvering purposes.